The dynamics of fuel cell stacks require that preferably fuel enters the stack from the opposite end of where the ambient air enters. As the fuel travels over cells, down the length of the stack, it is subject to electrochemical combustion. Typically a fraction of the unused fuel/combustion products mixture is recirculated, while a fraction is mixed with vitiated air to satisfy the mass balance. In prior art tubular solid oxide fuel cell (SOFC) generators, the SOFC geometry is cylindrical with one closed end. Fuel enters the stack at the cell closed end and flows upward in the space surrounding the cells. Air enters each cell through an air feed tube (AFT) concentrically positioned within the cell, exits the AFT at the closed end, and flows upward in the annular space between the AFT and the cell. As the fuel and air flow from the cell closed end to the open end, most of the fuel is electrochemically reacted with oxygen from the air producing electricity. The depleted fuel exiting the cell stack, which typically consists of 20% (H2+CO) and 80% (H2O+CO2), is combusted with the vitiated air exiting the cell in a combustion zone above the cell open ends to create exhaust gas. In this configuration no seals are required to separate the fuel stream from the air stream due to the closed end design of the tubular SOFC and the use of AFT's. A known construction for this type of fuel cell is described in U.S. Pat. No. 6,764,784.
In a known method of manufacturing closed end fuel cells, the interconnection between cells, the electrolyte and the fuel electrode layers are deposited on an extruded and sintered lanthanum manganite air electrode tube by plasma spray. The lanthanum chromite interconnection is in the form of a narrow strip that runs axially over the entire active length of the cell. A yttria stabilized zirconia electrolyte is deposited in such a way as to almost entirely cover the cell. The electrolyte layer overlaps the edges of the interconnection strip but leaves most of the interconnection exposed. Because the interconnection and electrolyte layers are dense, the overlap feature provides a seal that prevents direct mixing of air and fuel gas. A nickel/yttria stabilized zirconia cermet fuel electrode layer is deposited in such a way as to almost entirely cover the electrolyte, but leaves a narrow margin of electrolyte between the interconnection and the fuel electrode. This margin prevents shorting of the cell. Series electrical connections between cells are accomplished by means of a structure made from nickel foam and nickel screen. The foam part of which becomes sintered to the interconnection while the screen part becomes sintered to the fuel electrode of the adjacent cell. A solid oxide fuel cell according to this construction is described in U.S. Pat. No. 7,157,172, which patent is incorporated herein by reference.
In the above described closed end fuel cell designs, the closed end is a highly stressed region during plasma spray operations performed during manufacturing to deposit the interconnections, the electrolyte, and the fuel electrode. Elimination of the closed end may be expected to reduce failure due to thermal stress and thus significantly increase the yield during manufacturing. Furthermore, elimination of the air feed tubes would represent a substantial cost savings and a design simplification.
In an alternative construction for fuel cell stacks, a mid-sectional fuel distribution construction for fuel cells may be provide, as disclosed in US Patent Application Publication No. 2007/0087254, which patent application is incorporated herein by reference. In this construction, a fuel cell stack is described that comprises an air inlet, a series of fuel cells, a new fuel inlet, a fuel distributor, a recirculation plenum, and an exhaust. In all of the designs presented in this reference, fresh fuel from the fuel distributor enters the fuel cell stack in a middle-third section of the fuel cell stack, and the fresh fuel is divided to flow towards opposite ends of the stack. A common aspect of all of the designs presented in this reference is the fact that no seals are required to separate the fuel stream from the air stream.
It should be noted that in a typical closed end SOFC design, the combustion zone located near the entrance end of the air feed tubes operates to heat the entering air and improve the efficiency of the reaction. Accordingly, in alternative designs such as those that eliminate air feed tubes, it is important to ensure that the entry end of the fuel cell is maintained at a sufficient temperature to avoid negative effects on the cell performance.
There is a continuing need for a fuel cell stack construction that addresses problems associated with manufacturing of fuel cell stacks, while providing a high power density and increased operating efficiencies.